The present invention relates to an electrodynamic loudspeaker (speaker) and a magnetic circuit used for the speaker.
It is desirable for some acoustic apparatus to be very compact in size, and likewise, for speakers to be used in such apparatus to be compact and light in weight. Downsizing of the magnetic circuit is an effective approach for making the speakers small and light. So, magnets of Ndxe2x80x94Fexe2x80x94B system, which having a high energy product, have been increasingly used in the speakers. Since the Ndxe2x80x94Fexe2x80x94B system magnet is an expensive item, the magnetic circuit is required to have high efficiency. Meanwhile, when we look to the field of car-borne sound apparatus, among other fields, a full-range speaker which can reproduce sounds for the entire audible range is popularly used.
Now, a conventional speaker is described referring to FIG. 6.
The conventional speaker comprises a top plate 31, a magnet 32 disposed on an under plate 33 and vertically magnetized, a yoke 34, a voice coil 35 wound around a voice coil bobbin 36, a diaphragm 37, an edge 38, a damper 39, a frame 40, and a dust cap 41. There is a magnetic gap xe2x80x9caxe2x80x9d formed between the outer circumferential surface of the top plate 31 and the inner circumferential surface of the yoke 34, and the voice coil 35 is kept in the magnetic gap xe2x80x9caxe2x80x9d.
It is generally said, regarding the structure of magnetic circuits, that an inner magnet type circuit, for which a magnet 32 is disposed inside, has a higher efficiency; hence the inner magnet structure is advantageous in making a speaker compact and light in weight. However, in order to implement a high-efficiency magnetic circuit with the inner magnet structure, the diameter of the magnet 32 should be equal to that of the top plate 31 so that the magnetic flux coming from magnet 32 is directed only for the magnetic gap xe2x80x9caxe2x80x9d.
Furthermore, the gross weight of a vibrating system comprising a voice coil 35, a voice coil bobbin 36 and a dust cap 41 needs to be small if a speaker is intended to be compatible with full-range sounds. So, the voice coil 35 is naturally required to be light in weight.
For reducing the weight of a voice coil 35 while keeping the DC resistance constant, the voice coil wire is required to be small in the diameter and short in length. Consequently, the diameter of voice coil 35 becomes small in the full-range speakers.
The voice coil 35 is disposed in the magnetic gap xe2x80x9caxe2x80x9d, and generates a driving force in accordance with electric signals, which force is conveyed to the diaphragm 37 via the voice coil bobbin 36. In an inner magnet type full-range speaker using a high-efficiency magnetic circuit, the diameter of magnet 32 is smaller than that of voice coil 35. A magnet 32 of small diameter can deliver only a small amount of magnetic flux to the magnetic gap xe2x80x9caxe2x80x9d, which results in a smaller driving force produced by the voice coil 35. Namely, in the conventional inner magnetic speakers, the efficiency has been low, and they are unable to produce sufficiently great sounds.
On the other hand, in order to provide a sufficiently high driving force, the diameter of magnet 32 may be increased; but this results in a deteriorated efficiency. As the weight increases, so do the volume and the cost. If it is intended to increase the driving force by making the wire of voice coil 35 thicker and longer, there is the problem of increased weight with the voice coil 35. Thus it has been difficult to implement a full-range speaker of sufficiently high efficiency using a compact and light magnetic circuit.
The Japanese Patent Laid-open Publication No. H7-23498 discloses a speaker having an improved magnetic circuit, in which a center plate is sandwiched between two magnet pieces that magnetically repulse each other, in order to provide the magnetic gap with an increased density of magnetic flux. The assembly operation of the improved magnet circuit is conducted by unitizing the two magnet pieces already magnetized and repulsive to each other. So, inner diameters of the two magnet pieces and the hole provided in a center plate are made to be equal, and exclusive assembly jigs are used for manufacturing the speakers.
In the above-described improved magnetic circuit, however, the operating point of a magnet disposed on the top plate is low and the efficiency as a whole is reduced, because the two magnet pieces are made to have equal diameters, thicknesses and the like so that they are compatible with the assembly jigs. Furthermore, since the magnet is provided with a hole, the magnetic volume is decreased accordingly. Besides, exclusive jigs are needed for assembling a magnetic circuit, which leads to a complexity of the manufacturing process.
The present invention addresses the aforementioned drawbacks of the conventional speakers, and aims to provide a compact and light-weight full-range speaker that has a sufficiently high efficiency and can be assembled with ease during its production process. The present invention also provides a magnetic circuit for the speakers.
A speaker of the present invention comprises a magnetic circuit comprising a center plate, a first magnet and a second magnet disposed, respectively, on a lower surface and an upper surface of the center plate, the first magnet and the second magnet being magnetized so that they are magnetically repulsive to each other; an under plate disposed under the first magnet, and a yoke which forms a magnetic gap in relation to the outer circumferential surface of the center plate.
The speaker further comprises a voice coil bobbin having a coil kept in the magnetic gap, a diaphragm connected to the voice coil bobbin, an edge coupled with an outer circumferential edge of the diaphragm, and a frame.
The above magnetic circuit further comprises a top plate disposed on the upper surface of the second magnet. Both the second magnet and the top plate have a ring shape, an inner and an outer diameter of the two items being substantially the same. The outer diameter of the second magnet and the top plate is smaller than that of the center plate, and the center plate is provided with a protrusion on the upper surface for a positioning of the second magnet.
A clearance (i.e. play) may be provided between the protrusion and the second magnet at their contacting surfaces. The clearance is set so that the second magnet always stays within the region of the upper surface of the center plate even when a second magnet is disposed at a maximum eccentricity with respect to the center plate.
A speaker in another embodiment of the present invention comprises a center plate, a first magnet and an under plate, which are joined by a mechanical device such as a rivet, a bolt or the like to form a unitized member. The center plate and the under plate are provided respectively with a hole of substantially the same diameter (first diameter), and the first diameter is smaller than a hole diameter (second diameter) of the first magnet. The between the first diameter and the second diameter is determined so that the first magnet always stays within the region of the lower surface of the center plate, even when the first magnet is disposed at a maximum eccentricity with respect to the center plate.